Performance Study Of Fresnel Polar-axis Fixed-focus Concentrating Collector System

As one of the clean energy, solar energy is getting more and more worldwide attention and support of national policy. In solar thermal utilization, the most critical technology is to reduce system cost and improve the conversion efficiency of the solar thermal system. The Fresnel lens of the low-cost, low-light and high optical efficiency is combined with the cavity absorber of the low-cost and processing simply, which improve the conversion efficiency of solar thermal system on the basis of reducing the cost of concentrating collector system with good prospects. Aiming at shortcomings existing in the concentrating technologies, combined with a Fresnel lens and a cavity absorber advantages, this paper proposed a new Fresnel Polar-axis Fixed focus concentrating collector, and used a conical cavity as absorber, and built experimental bench of the concentrating collector system. The experimental system was employed to measure thermal properties, binding theoretical expressions of the heat removal factor and the thermal efficiency factor of the plate solar collector and the vacuum tube solar collector, the theoretical expressions of the heat removal factor and the thermal efficiency factor of the Fresnel Polar-axis Fixed-focus concentrating collector system were derived based on the cavity absorber, and using these two expressions to optimize the cavity absorber.In this paper, the experimental research was combined with the theoretical research, the main contents and conclusions are as follows:Firstly, the thermodynamic analysis of Fresnel concentrating solar collector was done based on the cavity absorber in the process of the thermal conversion, and in accordance with the second law of thermodynamics to gain the expressions of collector efficiency and exergy efficiency in the process. With this expression, we analyze the impact of the two main parameters of concentration ratio and collector temperature on collector efficiency and exergy efficiency, and gain the excellent collector temperature of the concentrating collector system under the different concentration ratio conditions.Secondly, in order to strengthen the advantages of the traditional concentrating collector and overcome its shortcomings. Based on the existing research results, this paper proposed the design of a new Fresnel Polar-axis Fixed-focus concentrating collector, and made the design process of the concentrating collector.Thirdly, the experimental bench of Fresnel Polar-axis Fixed-focus concentrating collector system using conical cavity absorber is built. The stagnation parameters, the instantaneous efficiency curves and the overall heat loss coefficient of the Fresnel solar concentrating collector system are obtained. The experimental data is analyzed. The following important conclusions are obtained:the Fresnel Polar-axis Fixed-focus concentrating collector system using conical cavity absorber has a stagnation parameter which is about 0.33 (m2·℃)/W; with the gradually increase of the average collector temperature, the solar thermal conversion efficiency of solar concentrating collector system declines gradually, The average collector temperature is about 30℃, The solar concentrating collector system has the most solar thermal conversion efficiency, which is about 54.08%; with the gradually increase of the average collector temperature, the overall heat loss coefficient of solar concentrating collector system increases gradually, The average collector temperature is about 30℃, The solar concentrating collector system has the least overall heat loss coefficient, which is about 281W/(m2·K).Finally, based on theoretical analysis and experimental investigation, the theoretical expressions of the heat removal factor and the collector efficiency factor of Fresnel Polar-axis Fixed-focus concentrating collector system using conical cavity absorber are derived. The theoretical value of the heat removal factor and the collector efficiency factor are calculated by using these theoretical expressions, and compared with the experimental value of the heat removal factor and the collector efficiency factor; Using these theoretical expressions optimize the design of the conical cavity absorber. The following important conclusions are obtained:the error factor is within ±7% between the theoretical values and the experimental values of the heat removal factor and the collector efficiency factor; There is an optimal diameter of the cavity aperture, which is about 100mm; An optimal vertex angle of cross section is about 60°-80°; In order to obtain a good performance of the collector, the convection heat transfer coefficient of Fresnel Polar-axis Fixed-focus concentrating collector system should be more than700W/(m2·K), the overall heat loss coefficient should be less than 300W/(m2·K).